Want FCU to run only when boiler flame is lit

[desktop987456]

Hi,
I have a Fan Coil unit in lounge and radiators in Bedrooms and Bathrooms. I also have a wired thermostat in Lounge with the boiler which is set to 19degC.

Issue - FCU never turns off, it keeps running, if boiler is firing, FCU will blow hot air but when Boiler trips and as water temperature cools down, FCU starts to blow cold air (which is counter productive in maintaining the Lounge temperature; I have to manually switch the FCU off and then turn back on when boiler fires again).

I know FCU works best at high water temperatures whereas I am running at a boiler flow temperature of 50DegC which works fine for radiators in the property (I have oversized rads).

Remedy - I was thinking if I put a pipe thermostat (like attached) on FCU flow pipe and set the Pipe thermostat to 36DegC so it turns off the FCU when return temperature falls below 36DegC.

Would this approach work for FCU in what I want to do or is there a better solution.

Thanks for taking time to read the posting.

 

[Harry Bloomfield]

Remedy - I was thinking if I put a pipe thermostat (like attached) on FCU flow pipe and set the Pipe thermostat to 36DegC so it turns off the FCU when return temperature falls below 36DegC.

I would install the pipe stat between FCU and the fan's motor, maybe inside the unit itself. I'm surprised there is not such a pipe stat built into the unit already?

 

[desktop987456]

No, there isn't, its an older unit.

 

[desktop987456]

I installed an old dial type thermostat inside the FCU, I think it is called a bimetallic stat.

I checked the thermostat by dipping into hot water first but it doesn't activate the fan, once it is cooled down only then it will activate the fan, is it suppose to work like this? I thought I want it to activate FCU when water is warm enough to give hot air.

 

[Harry Bloomfield]

I checked the thermostat by dipping into hot water first but it doesn't activate the fan, once it is cooled down only then it will activate the fan, is it suppose to work like this?

You need a pipe stat, where the contacts close on rise in temperature and the pipe stat, needs to be mounted on the flow pipe into the radiator.

Some stats, close on rise, some close on fall, some have both options.